Journal Article

Black hole mergers: do gas discs lead to spin alignment?

Giuseppe Lodato and Davide Gerosa

in Monthly Notices of the Royal Astronomical Society: Letters

Published on behalf of The Royal Astronomical Society

Volume 429, issue 1, pages L30-L34
Published in print February 2013 |
Published online November 2012 | e-ISSN: 1745-3933 | DOI: http://dx.doi.org/10.1093/mnrasl/sls018

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In this Letter, we revisit arguments suggesting that the Bardeen–Petterson effect can coalign the spins of a central supermassive black hole binary accreting from a circumbinary (or circumnuclear) gas disc. We improve on previous estimates by adding the dependence on system parameters and noting that the non-linear nature of warp propagation in a thin viscous disc affects alignment. This reduces the disc's ability to communicate the warp, and can severely reduce the effectiveness of disc-assisted spin alignment. We test our predictions with a Monte Carlo realization of random misalignments and accretion rates, and we find that the outcome depends strongly on the spin magnitude. We estimate a generous upper limit to the probability of alignment by making assumptions which favour it throughout. Even with these assumptions, about 40 per cent of black holes with a ≳ 0.5 do not have time to align with the disc. If the residual misalignment is not small and it is maintained down to the final coalescence phase, this can give a powerful recoil velocity to the merged hole. Highly spinning black holes are thus more likely being subject to strong recoils, the occurrence of which is currently debated.

Keywords: accretion, accretion discs; black hole physics; galaxies: evolution; galaxies: nuclei

Journal Article.  4082 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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